linux-kernel - Re: [RFC Patch] use MTRR for write combining if PAT is not available

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Date:	Thu, 22 Oct 2009 06:35:50 -0700
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Thomas Schlichter <thomas.schlichter@....de>
Cc:	Jan Beulich <JBeulich@...ell.com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	"dri-devel@...ts.sourceforge.net" <dri-devel@...ts.sourceforge.net>,
	Robert Hancock <hancockrwd@...il.com>,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	"H. Peter Anvin" <hpa@...or.com>,
	"jbarnes@...tuousgeek.org" <jbarnes@...tuousgeek.org>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Thomas Hellstrom <thellstrom@...are.com>,
	Tejun Heo <tj@...nel.org>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	"x86@...nel.org" <x86@...nel.org>, Yinghai Lu <yinghai@...nel.org>
Subject: Re: [RFC Patch] use MTRR for write combining if PAT is not
 available

On Thu, 2009-10-22 at 05:08 -0700, Thomas Schlichter wrote:
> When forking, what happens with the "struct file"? If it is being copied, then the
> processes share the same private data which would be freed during the first
> release(). I think this would be a problem whereever file-private data are used.
> 
> So I think it must be shared between the forked processes and some reference
> counting must exist. This reference counting must ensure that release() is only
> called when all processes did close() their file.
> 
> And in that case (shared "struct file", one single release() call in the end) this
> implementation should be completely safe...

I am referring to the refcount getting incremented. Also, let me think
about your direction (as the pci_mmap_page_rane() is explicitly adding
the mtrr, we should perhaps do the ref counting there, perhaps)

> > There is no need to go through num_var_ranges etc.
> 
> Well I have to remember wich file added which MTRR entries. Because I have
> to remove them if the file is being closed. Therefore I need an array of size
> "num_var_ranges" (or MTRR_MAX_VAR_RANGES which is the uper bound).

No. the private data  for example can keep track of a struct containing
mtrr number and ref count etc. Exporting var_ranges and going through
var ranges elements in an array is not clean, especially when you are
populating only one element.

thanks
suresh

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